2, No. 4, 1963-1973. e- ISSN: 2249 â1929. Journal of Chemical, Biological and Physical Sciences. An International Peer Review E-3 Journal of Sciences.
August-October, 2012, Vol. 2, No. 4, 1963-1973.
e- ISSN: 2249 –1929
Journal of Chemical, Biological and Physical Sciences An International Peer Review E-3 Journal of Sciences
Available online at www.jcbsc.org Section C: Physical Sciences CODEN (USA): JCBPAT Research Article
Structural, electrical and photoluminescence Properties of In2O3-Doped SnO2 Mahdi Hassan Suhail11, Manal Madhat Abdullah11 & Sabah Ibrahim Abbas 2 1
Department of Physic, College of Science, University of Baghdad, Iraq
Department of Physics, College of science, University of Wassit, Iraq
Received: 1 July 2012; Revised: 29 July 2012 Accepted: 4 August 2012
ABSTRACT Undoped and doped SnO2 with In2O3 thin films were prepared by using the thermal spray pyrolysis method from SnCl2.2H2O and InCl3 dissolved in isopropyl mixing with water solution (1:1) on the glass substrate heated at 400°C - 450 °C. The structure of In2O3-doped SnO2 thin films were investigated by X-ray diffraction patterns. The morphology and crystallite size was evaluated by using Atomic Force Microscope (AFM).The band gap energy was 3.85 eV for pure SnO2 .The photoluminescence spectrum of these samples were an excited at wavelength 270 nm and consist of the strong emission band located at 415 nm, and 533 nm.
Keywords: SnO2 Thin films; spray pyrolysis; activation energy; photoluminescence structural and electrical of SnO2, .
INTRODUCTION Transparent conducting oxide (TCO) films have been widely used as a thin film material for application in various fields such as gas sensors, optoelectronic devices, flat panel displays, heat mirrors, and solar cells 1-5. The increased utilization of many transparent electrodes has recently accelerated the development of inexpensive TCO materials. Indium-doped tin oxide (ITO) film is a generally preferred TCO material for these applications owing to its excellent electrical and optical properties 6. SnO2 film shows the best thermal and chemical stabilities. Also, it is inexpensive to make and has good mechanical durability, but has a high resistivity. SnO2 is an n-type semiconductor with a wide band gap of approximately 3.7 eV. Pure SnO2 films are poor electrical conductors that are highly transparent in the 1963 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.3, 1963-1973.
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visible range. However, their poor electrical conductivity can be improved by controlling the stoichiometry or doping with impurities. Conductive indium-doped tin oxide (ITO) films are prepared by various methods such as spray pyrolysis , sputtering, and evaporation 9-12. Chemical spray pyrolysis has advantages among these methods, Spray pyrolysis is one of the simplest deposition techniques employed in various kinds of thin films due to its simplicity, and its compatibility with large area coatings without high vacuum ambience. Furthermore, the capital cost and the production cost of high quality metal oxide thin films is lowest for sprayed thin films. Moreover, the spray pyrolysis technique is well suited to controlling the texture via the tuning deposition temperature and mass production capability for uniform large area coatings. Vasu et al 13 and Shanthi et al 14 , they have observed that the nozzle-to-substrate distance (NSD) ,substrate temperature and air flow rate play a significant role in determining the size distribution of the droplets in the pyrolytic reaction, whether it is homogeneous or heterogeneous. It is known that the homogeneous reaction (which affects, the conductivity and the visible transparency of the SnO2, films) can be minimized by adjusting the substrate temperature and NSD. They were reported that the NSD and by suitably adjusting these parameters, it is possible to force a heterogeneous reaction to take place in the vapor phase. 7-8
This study focuses on prepared undoped SnO2 thin films and doped with In2O3 thin films with different concentration (3, 5 and7%) by chemical spray pyrolysis and study the effect of In2O3-additivated on structural, morphological, optical properties and additivated effect on intensities of photoluminescence (PL) measurements of sprayed Indium doped SnO2 thin films.
EXPERIMENTAL Preparation undoped and doped SnO2 with indium Oxide thin films ( Pure , 3 at %, 5 at %, 7 at %) were deposited using an aqueous – isopropanol solution including SnCl2.2H2O (99.8%, Aldrich) and InCl3 (98%) (0.1 M), H2O and CH3CH2OH (1:1) by the spray pyrolysis technique on the glass substrates at temperatures ranging from 400 °C - 450°C, which is known to be the optimal range for the formation of SnO2 films 15. The solution was sprayed at the following conditions: carrier-air pressure: 1-2 atm., flow rate of solution: 9 ml/min, and substrate-to-nozzle distance: 35 cm. The metallic salt solution, when sprayed onto a hot substrate, pyrolitically decomposes and a chemical reaction takes place on the heated substrate and a thin layer of SnO2 is deposited. The phenomenon for the preparation of a metal oxide thin film depends on surface hydrolysis of metal chloride on a heated substrate surface in accordance with the equation16 XClm + nH2O → XOn +mHCl Where X is the metal such as Sn, In, Co … etc of the oxide films. The structural of the thin films were examined by X-ray diffractometer (6000-Shimadzu) using CuKα radiation with a wavelength, λ=1.54060 Å. The morphological of the films were analyzed using scanning Probe Microscope (SPM, model AA3000 Angstrom Advanced .Inc). The optical absorbance of the films was measured using UV-visible spectrophotometer (SP-3000 Optima) in the wavelength range 200-1200 nm at room temperature. The deposition of aluminum electrodes using masks where the finger width is 1000 nm and distance between two fingers is 1000 nm. The photoluminescence measuring for range (300-900 nm) by (Spectro Fluorometer SL 174, Elico, India).
RESULTS AND DISCUSSION The typical XRD spectra of undoped and doped SnO2 films with thickness (534 nm for Pure, 975nm for 3%, 1335 nm for 5% and 652 nm for 7%) are shown in Figure 1. 1964 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.4, 1963-1973.
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Fig.1: XRD crystal structure of In2O3 – doped SnO2 (Pure, 3%,5%,7%) thin films on glass substrate at 400 .
All of the indium oxide doped SnO2 samples were nanocrystalline in nature. At the higher doping ratio of Indium oxide peaks become evident, indicating that it has formed a separate phase. Indium is much closer in size to a tin atom and will more easily substitute and dope into the SnO2 crystal lattice 17. XRD data showed that the tetragonal form of SnO2 has formed by chemical spray, Dominant peak at (110) parallel to the substrate. The rutile crystal structure of tin oxide has a tetragonal unit cell with the lattice constants are a=b=4.731 Å and c =3.189 Å. A few extra peaks not associated with the cassiterite structure appear to be beginnings of a not fully formed In2O3 (bixbyite) structure 18. The intensity of other peaks is small; it indicates that our films are textured. At that, the degree of the texturing depends on kind of sprayed solution we used, they indicate that the change of predominant orientation of crystallites, confirms the cassiterite structure of nanocrystalline SnO2 19. The (110) is the dominant crystal structure of the low-index crystal faces for this material due to its stability. This is the desired structure of SnO2 for sensing applications since its prevalent (110) growth plane is extremely stable and can reject oxygen with little distortion 20. Growth of this plane helps in achieving high oxygen vacancy concentrations at low temperatures. The result is in a good agreement with data mentioned in the literature (JCPDF card no 36-1451)21. Wideness of the peaks indicates that SnO2 films are composed of small nanoparticles as shown in Figure 1 22.
1965 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.4, 1963-1973.
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AFM can be used to study the surface morphology with resolution is 0.1 nm. AFM images of the as asdeposited undoped and doped SnO2 with In2O3 films dissolved in Water and Isopropyl (1:1) ratio deposition at 450 on glass substrate are shown in Figure 2.
Fig. 2: AFM image of undoped and doped SnO2 thin films deposited on glass substrates at temperature 450°C. The grain size and RMS roughness of these films is shown in Table .1. The AFM images of all samples displayed a granular structure. The granular films show higher surface area, which is conducive for film film23 gas interaction and results in higher sensitivity . The grain size is decreased with increased doping ratio. In the undoped SnO2 a coarse and irregular surface with low grain density distribution and these Grains are not tightly packed is evident. The small spherical grains agglomerates are shown in the figure. The smooth enough due to optimization of the deposition con conditions ditions such as distance between the nozzle and 22 the substrate . A lower surface roughness with uniform oriented for 3% doping, this may correspond to the columnar structure which is associated with the (110) SnO2 textured growth 24. The grain density 1966 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.4, 1963-1973.
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decreases and the roughness of these films are increase for (5%, 7%) due to the existence of many hillocks, which are faceted and distributed randomly on the relatively smooth surface 25. Table.1: Physical parameters of grown undoped and doped SnO2 thin films on glass substrate.
Average Roughness Crystallite Average sizes from (nm) AFM (nm)
Activation energies (eV)
Conductivity x10-2(Ω Cm)-1
SnO2: In2O3 350 (3 wt.%) SnO2: In2O3 373 (5 wt.%) SnO2: In2O3 178 (7 wt.%)
Ea1 Ea2 Ea1 Ea2 Ea1 Ea2 Ea1
0.301 0.077 0.689 0.052 0.526 0.029 0.209
1.84 3.15 1.80
The optical transmittance was measured by UV-Vis spectrophotometer for undoped and doped SnO2 films (wavelength ranging from 200-1100 nm) were deposited at 450 on glass substrate. Figure 3 shows the optical transmittance spectra of thin films.
100 90 80 70 60 50 40 30 20 10 0
pure %3 %5 %7
680 880 1080 Wavelength (nm)
Fig.3: Optical transmittance of undoped and doped SnO2 thin films deposited on a glass substrate at 450 °C.
All undoped and doped SnO2 films that dissolved in Isopropyl alcohol and water (1:1) demonstrate more than 83% transmittance at wavelengths longer than 380 nm (comparable with the values for the ITO thin films prepared by Chacko 26). A sharp decrease in the transmittance of the films at wavelength below 350 1967 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.4, 1963-1973.
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nm is probably due to the absorption edge in this region which can also manifest the crystalline nature of the SnO2 films. An increase in the transmittance of the films (pure) could be attributed to the decrease in free carriers 22. Transmittance spectra of the samples also show a dependence on the variation of lattice parameters. The increase in transmittance and conductivity can be attributed to the improved crystalline structure. The reduction of population density of oxygen vacancies that enhanced the crystalline nature of the films. The transmittance decreases to 83% for (3%, 5% and 7 %) due to increase the proportion of impurities. This can be explained as that caused by an increase in lattice disorder due to increased population density of oxygen vacancies which in turn reduces the crystalline nature of the films 26. Figures 4 Shows the variation of against h for all samples. The value of Eg equal 3.85 eV for pure SnO2 corresponds to the Drake 27. The Moss-Burstein effect (shift in band-gap) results from the Pauli Exclusion Principle and is seen in semiconductors as a shift with increasing doping of the band-gap as defined as the separation in energy between the top of the valence band and the unoccupied energy states in the conduction band 28. Band gap values increase slightly for high doping concentration due to improved structure and or enhanced quantum confinement. An oxygen-deficient film usually has a wide band gap, resulting in a blue shift of the optical transmission spectrum. This shift is due to the increase in the carrier concentration, which shifts the Fermi level towards the conduction band and enhanced the magnitude of the Burstein-Moss effect. It should be noted here that at very high carrier densities for 7% doping the electron-electron and electron-impurity scattering could cause a band-gap narrowing 29. Conductivity is measurement by Hall Effect Measurements with high conductivity for p- type samples (5%), than n-type samples (pure, 3% and 7%) as shown in Table.1. The conductivity of undoped and doped SnO2 is due to defects such as oxygen vacancies, lattice disorders. etc that result from incomplete oxidation of the films. Electrical conductivity is increase with concentration of oxygen vacancies. The variation in conductivity with the change in thin film deposition substrate temperature can be explained as due to the concentration of oxygen vacancies and local lattice disorder30. The activation energy in the lower temperature region is always less than the energy in the higher temperature region because material passes from one conduction mechanism to another. In low temperature region, the increase in conductivity is due to the mobility of charge carriers which is dependent on the defect/dislocation concentration. So, the conduction mechanism is usually called the region of low temperature conduction.
2E+11 pure %3 %5 %7
(αhʋ ʋ )^2 ( cm-2 eV2)
1.6E+11 1.2E+11 8E+10 4E+10 0 2
2.4 2.8 3.2 3.6
hʋ ʋ (eV)
Fig.4: Plots of (αhν)2 vs. photon energy of undoped and doped SnO2 thin films deposited on glass substrates at temperature 450°C. 1968 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.4, 1963-1973.
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In this region activation energy decreases, because a small thermal energy is quite sufficient for the activation of the charge carriers to take part in conduction process. Hence increase in the conductivity in the lower temperature region can be attributed to the increase in charge mobility. In high temperature region, the activation energy is higher than that of low temperature region. In this region the electrical conductivity is mainly determined by the intrinsic defects and hence is called high temperature or intrinsic conduction. The high values of activation energy obtained for this region may be attributed to the fact that the energy needed to form defects much larger than the energy required for its drift31. The plots of ln(σ) versus 103/T in the range (323 K-523 K) for undoped and doped SnO2 with In2O3 at different concentration which are deposition at 450 on glass substrate are shown in Figure 5.
2LN(σ) (Ω cm)-1
Ln(σ) (Ω cm)-1
2.5 1000/T (1/◦K)
1000 /T (1/◦k)
(5%) LN (σ) (Ω cm)-1
LN(σ) (Ω cm)-1
1000/T (1/ ◦K)
Fig.5: Plot of ln (σ) verses 1000/T for undoped and doped SnO2 with In2O3 for (Pure, 3%, 5%, 7%).
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Photoluminescence of the undoped and doped SnO2 deposition on the glass substrate at an excitation wavelength of 270 nm is investigated at room temperature and the result is shown in Figure 6.
800 Intensity (a.u)
Wave length( nm)
600 400 200 0
400 500 Wave length (nm)
400 500 Wave length(nm)
Fig.6: Photoluminescence spectrum of the undoped and doped SnO2 with In2O3 on glass substrate for(Pure ,3%,5%,7%). The PL spectra consist of the strong emission band located at 415 nm, and 533 nm. Since the energy gap of the SnO2 bulk is 3.62 eV, the band-to-band emission peak at 326 nm of undoped and doped SnO2 was not observed due to the limitation of the PL detection range. The appearance of the 415 nm peak is independent of the concentration of oxygen vacancies, while due to structural defects or luminescent centers, such as nanocrystals and defects in the SnO2. Peaks could be attributed to the band-acceptor and (415nm) donor-acceptor pair transitions, respectively, which concern structural defects or impurities formed during the growth 32. The 530 nm peak is attributed to the electron transition mediated by defect levels in the band gap, such as oxygen vacancies 33. These oxygen vacancies are created as the donor level below the conduction band and are the origin of the n-type SnO2 nanostructures. The intensity of this peak increases with the increasing concentration of oxygen 34. Those two emission bands typically occur: a band-edge luminescence band (near-UV region 410nm) and a below-band gap band in the green region peaked at about 530 nm( it has been attributed to transition between photo excited holes and singly ionized oxygen vacancies, to antisite oxygen, to donor–acceptor complexes or to surface states35). This latter green emission characterizes the nanostructured systems and is not observed in bulk SnO2 wafers. Some results reported in literature evidences that the green luminescence band is closely related to surface electrons recombination. It should be noted that the PL spectra also showed that when In2O3 atoms were doped into SnO2 nanostructures, the intensity of PL luminescence would decrease significantly as compared with Mariammal et al 36. 1970 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.4, 1963-1973.
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The decrease in UV emission was attributed to the increased intrinsic defects of nanoparticles when introduction of doping indium. The emission spectral intensity increased of Indium doped (5 at. %) SnO2 nanopartical behavior may be attributed to an increase in the number of luminescence centers by increasing the ratio of surface area.
CONCLUSIONS We have successfully prepared undoped and doped SnO2 with In2O3 films by spray Pyrolysis method, the Crystallite sizes and Roughness was decrease with doping by indium oxide. Electrical properties shown that there are two activation energy for (Pure, 3% and 5%) and one activation energy value for (7%). The activation energy increase with doping for 3% and 5%. The PL spectra consist of the strong emission band located at near-UV region 410nm and peaked at about 530 nm green region. The PL spectra also showed that when In2O3 atoms were doped into SnO2 the intensity of PL luminescence would decrease significantly.
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*Correspondence Author: Mahdi Hassan Suhail1; Department of Physic, College of Science, University of Baghdad, Iraq
1973 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.4, 1963-1973.